Antagonistic Coevolution

Because of the Red Queen Hypothesis, certain species that can reproduce sexually and asexually will choose sexual reproduction when there are parasites are present. This is true of certain shell-bearing snails that get parasitized by trematodes. If there are parasites, there is an advantage to doing sexual reproduction because the offspring might be better fit to fight off the parasite. The Red Queen hypothesis was named after Lewis Carroll s story in which there was constant running, even though there was no increase in actual distance covered.

ANTAGONISTIC COEVOLUTION

Antagonistic coevolution involves the relationships between males and females of a species so that there are changes in sexual structures to counteract those of the opposite sex in order to have maximal reproductive success. Male mating behavior is often detrimental to the female s fitness, particularly in those species that have traumatic insemination. Females must change in order to maintain reproductive success. This is what happens in certain insects that, if their thoraxes are inseminated too many times, they will die.

Many female insects will be damaged by frequent male inseminations so those that survive can reproduce. Females have a spermatheca or pseudo-spermatheca that play a role in sperm storage. Females can use the stored sperm to inseminate themselves when the circumstances are optimal. Not all sperm get used for insemination and some get expelled. Females also have enzymes that break down seminal fluid in order to select the sperm to be chosen. Females can also be evasive and will fend off insemination attacks.

Males have developed spiny genitalia that will help anchor them during copulation. This can scar the female reproductive tract so females have developed thicker copulatory tracts. Males have also developed a method to inseminate the female abdomen directly. This is called traumatic insemination. Females have evolved to phagocytize or kill the sperm cells. Males will develop a longer sperm tail length because the females seem to prefer this particular trait. There is no other evolutionary reason known for this evolutionary tail length discrepancy.